linux-kernel - Re: [PATCH/RFC v3 5/5] media: Add registration helpers for V4L2 flash sub-devices

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Message-id: <536C815F.8060906@samsung.com>
Date:	Fri, 09 May 2014 09:18:55 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Sakari Ailus <sakari.ailus@....fi>
Cc:	linux-media@...r.kernel.org, linux-leds@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	s.nawrocki@...sung.com, a.hajda@...sung.com,
	kyungmin.park@...sung.com
Subject: Re: [PATCH/RFC v3 5/5] media: Add registration helpers for V4L2 flash
 sub-devices

Hi Sakari,

On 05/07/2014 09:58 AM, Sakari Ailus wrote:
> Hi Jacek,
>
> On Wed, May 07, 2014 at 09:20:17AM +0200, Jacek Anaszewski wrote:
>> On 05/06/2014 11:10 AM, Sakari Ailus wrote:
>>> Hi Jacek,
>>>
>>> On Tue, May 06, 2014 at 08:44:41AM +0200, Jacek Anaszewski wrote:
>>>> Hi Sakari,
>>>>
>>>> On 05/02/2014 01:06 PM, Sakari Ailus wrote:
>>>>
>>>>>>>> [...]
>>>>>>>>>> +static inline enum led_brightness v4l2_flash_intensity_to_led_brightness(
>>>>>>>>>> +					struct led_ctrl *config,
>>>>>>>>>> +					u32 intensity)
>>>>>>>>>
>>>>>>>>> Fits on a single line.
>>>>>>>>>
>>>>>>>>>> +{
>>>>>>>>>> +	return intensity / config->step;
>>>>>>>>>
>>>>>>>>> Shouldn't you first decrement the minimum before the division?
>>>>>>>>
>>>>>>>> Brightness level 0 means that led is off. Let's consider following case:
>>>>>>>>
>>>>>>>> intensity - 15625
>>>>>>>> config->step - 15625
>>>>>>>> intensity / config->step = 1 (the lowest possible current level)
>>>>>>>
>>>>>>> In V4L2 controls the minimum is not off, and zero might not be a possible
>>>>>>> value since minimum isn't divisible by step.
>>>>>>>
>>>>>>> I wonder how to best take that into account.
>>>>>>
>>>>>> I've assumed that in MODE_TORCH a led is always on. Switching
>>>>>> the mode to MODE_FLASH or MODE_OFF turns the led off.
>>>>>> This way we avoid the problem with converting 0 uA value to
>>>>>> led_brightness, as available torch brightness levels start from
>>>>>> the minimum current level value and turning the led off is
>>>>>> accomplished on transition to MODE_OFF or MODE_FLASH, by
>>>>>> calling brightness_set op with led_brightness = 0.
>>>>>
>>>>> I'm not sure if we understood the issue the same way. My concern was that if
>>>>> the intensity isn't a multiple of step (but intensity - min is), the above
>>>>> formula won't return a valid result (unless I miss something).
>>>>>
>>>>
>>>> Please note that v4l2_flash_intensity_to_led_brightness is called only
>>> >from s_ctrl callback, and thus it expects to get the intensity aligned
>>>> to the step value, so it will always be a multiple of step.
>>>> Is it possible that s_ctrl callback would be passed a non-aligned
>>>> control value?
>>>
>>> In a nutshell: value - min is aligned but value is not. Please see
>>> validate_new() in drivers/media/v4l2-core/v4l2-ctrls.c .
>>>
>>
>> Still, to my mind, value is aligned.
>>
>> Below I execute the calculation steps one by one
>> according to the V4L2_CTRL_TYPE_INTEGER case in the
>> validate_new function:
>>
>> c->value = 35000
>>
>> val = c->value + step / 2;       // 35000 + 15625 / 2 = 42812
>> val = clamp(val, min, max);      // val = 42812
>> offset = val - min;              // 42812 - 15625 = 27187
>> offset = step * (offset / step); // 15625 * (27187 / 15625) = 15625
>> c->value = min + offset;         // 15625 + 15625 = 31250
>>
>> Value is aligned to the nearest step.
>>
>> Please spot any discrepancies in my way of thinking if there
>> are any :)
>
> min is aligned to step above. This is not necessarily the case. And if min
> is not aligned, neither is value.
>

Thanks for spotting this. Below are improved versions of the conversion
functions. Please let me know if you have any comments.

static inline
enum led_brightnessv4l2_flash_intensity_to_led_brightness(
                                         struct led_ctrl *config,
                                         u32 intensity)
{
         return ((intensity - config->min) / config->step) + 1;
}

static inline
u32 v4l2_flash_led_brightness_to_intensity(
                                         struct led_ctrl *config,
                                         enum led_brightness brightness)
{
         return ((brightness - 1) * config->step) + config->min;
}

Regards,
Jacek Anaszewski
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